Devices using electromagnetic waves having a frequency in the terahertz domain (e.g. from 1 to 100 THz) have important applications, for example in spectroscopy, medicine, security and imaging. These devices include terahertz detectors/transducers operating in the terahertz domain which rely on the conversion of the radiation energy of an incident electromagnetic wave into a measurable form.
For example, in thermal terahertz detectors/transducers, the energy of the incident electromagnetic wave is absorbed and converted into heat. The efficient thermal detection of terahertz electromagnetic waves has technologically developed around two main concepts: bolometers and Golay cells.
As an example, some bolometers convert the energy of the incident electromagnetic wave to heat-up a small resistance made of for example highly doped silicon or germanium. The small change in the resistance value due to the heating is detected by an electrical circuit, i.e. by detecting a bias change on the resistor. These kind of bolometers achieve a minimal detectable power from the detector, in other words a Noise Equivalent Power (NEP) sensitivity of 1 pW/sqrt(Hz) at 4 Kelvins, with a frequency response up to 50 kHz.
In other types of bolometers, the resistance is made of Niobium, which becomes a superconducting material when cooled below a critical temperature of 10 K. Such bolometers are operated just at the superconductor's critical temperature and rely on photon absorption. When a photon is absorbed, the bolometer switches to a normal state and produces a large change in resistance. Such bolometers achieve a Noise Equivalent Power (NEP) sensitivity of 1 pW/sqrt(Hz), with a bandwidth of up to 1 kHz, and operate at a cryogenic temperature of 8 Kelvins.
Other types of bolometers are Indium Antimonide (InSb) bolometers that, in combination with applied magnetic fields and cryogenic operation at a temperature of 4K, achieve detection with NEP=0.5 pW/sqrt(Hz) at frequency up to 500 kHz. All these bolometers, doped silicon or germanium, Niobium, and Indium Antimonide, can be operated in the electromagnetic frequency band 1-20 THz. However, they operate only at cryogenic temperatures (<10K).
In Golay cells, which operate in the range 1-50 THz, the terahertz radiation is transformed into heat thanks to an absorbing element placed within a gas-filled enclosure. The resulting increase in gas pressure pushes a flexible membrane, whose displacement may be detected optically. Being based on thermal effects and thermal distortion of the membrane, the response time is long by comparison with bolometers, but a Golay cell can operate at room temperature. Some commercial Golay cells achieve THz waves detection with 140 pW/sqrt(Hz) of sensitivity, up to a frequency of 20 Hz.
All these techniques have the drawbacks of operating at low temperature, having a long response time, or not operating at all frequencies within the terahertz domain. There is therefore a need for methods or devices allowing electromagnetic waves to be detected or transduced within the entire terahertz domain, at room temperature, with high sensitivity, with short response time, while maintaining the possibilities of miniaturization and integration.
The patent application WO2006/055961A1 discloses a sensor for detecting high frequency signals.